Longevity briefs provides a short summary of novel research in biology, medicine, or biotechnology that caught the attention of our researchers in Oxford, due to its potential to improve our health, wellbeing, and longevity.
Why is this research important: Fasting is a healthy lifestyle practice that promotes weight loss, lowers the risk of age-related diseases, and it has been speculated (though not proven) that fasting may extend lifespan in humans. However, fasting is also difficult, especially periodic fasting – that is to say complete abstinence from food for an extended period, usually 48 hours or longer. Some are likely to be put off fasting for fear of lacking mental and physical energy, and perhaps reduced ability to work effectively. Previous studies have suggested that fasting doesn’t significantly affect cognitive performance. However, there are many different approaches to fasting, and it may be beneficial to know if some forms of fasting affect cognition or energy levels differently.
What did the researchers do: In this study, researchers conducted a randomised crossover trial of three different fasting methods. This means that participants were randomised to follow one of three fasting diets. Then, after a ‘washout period’, each group was swapped over to a different fasting method until all participants had completed all three fasting experiments. In total, 18 women aged 21 to 49 years old participated in the study. Their average body mass index (BMI) was 25.8 (a BMI of above 25 is considered overweight).
During each experiment, participants had their calorie intake reduced for 7.5 hours. One group ate no food at all (complete fasting). One group ate two liquid meal replacements totalling 512 calories in the morning (one at 9:30am and one at 11:30am). The third group ate one liquid meal replacement in the morning (9:30 am) and two snack bars in the afternoon (12:30pm and 3:30pm) totalling 522 calories. Researchers measured participants’ cognitive performance using three cognitive tasks. They also made other measurements, including participants’ blood sugar, and participants were asked to report their subjective fatigue.
Key takeaway(s) from this research: Cognitive performance didn’t significantly change over the course of the day in any of the groups. This is in agreement with past research, including a meta-analysis of fasting studies from 2014, which found no significant relationship between fasting and cognitive performance. The group undergoing complete fasting experienced higher levels of mental fatigue and hunger than the other two groups, and experienced a greater decrease in blood sugar.
The fact that the experiments in this study lasted only 7.5 hours limit their relevance to real-world scenarios, because 7.5 hours is a relatively short fast duration and is unlikely to have a significant health impact. Most fasting diets aim to either maintain a fasting state over the course of several days (periodic fasting), or to fast for shorter periods, but to do so repeatedly every day for an extended period (intermittent fasting).
With this limitation in mind, the study supports previous evidence that fasting doesn’t impact cognitive performance, but suggests that fasting practices that don’t completely eliminate calorie intake are less unpleasant. Concerning this point, it’s worth noting that participants of this study were all women, and that some evidence suggests that there are sex differences in satiety and hunger responses. Specifically, studies have suggested that men find complete elimination of calories to be less unpleasant than calorie restriction.
Modified Fasting Compared to True Fasting Improves Blood Glucose Levels and Subjective Experiences of Hunger, Food Cravings and Mental Fatigue, But Not Cognitive Function: Results of an Acute Randomised Cross-Over Trial: https://doi.org/10.3390/nu13010065